What do people buy 3D models for?
The main reason is because it's cheaper to buy the final model than it is to pay an artist to create the model. Since the model can be sold any number of times as well, the original creator benefits despite selling it for less than it cost them to make. I've used models that cost over \$200, but would otherwise take well over a week to create, and that's assuming I could find all the references needed to create the model.

As to the types of models and their uses, that depends on the situation entirely. I would imagine a lot of the models like buildings are used as backgrounds for ads or commercials, or as background models for animations. Depending on the situation as well, the models could be used as the focus of the animation, but as you mentioned that is pretty rare as finding exactly what you need can be tough. 

I can (try to) lay out a hypothetical situation to make things more clear. Say a company wants a 30 second ad created involving a flyover of a city and the camera then drops to a parking lot where something happens. Assuming none of this can be filmed in the real world, there's a lot that needs to be created. Let's say that it would take a single artist 2 weeks to create the entire shot from scratch. If the artist gets paid \$200 a day (probably on the low side) and works 6 day weeks, you have a total cost of \$2,400. That's not including any textures or references that may need to be bought. Now lets say there is a 3d city scene available for \$500, plus a parking lot scene for \$200, And a fleet of cars for the lot for \$400. Some background images could be used to speed things up, adding \$50, and some sky HDRi's for \$100. We're now sitting at \$1,250, but there's probably a couple days work for the artist to add it all together and create the final elements or modify the bought ones. Say that adds another \$400. We can round the total cost to be \$1,500, or \$900 less than paying the artist to do everything. This isn't entirely accurate of course, as it's just some random numbers and times, but the point is that despite the costs of the models, which can seem high, time is money, so it's a matter of what is the cheapest.